The car struck a light pole, two trees and a sign noting the street's 45 mph speed limit, but Los Angeles County Sheriff's detective Jeff Maag tells PEOPLE the driver "was doing well over 45 – [it's] fair to say at least twice that."

Maag says it will take days or even weeks to complete the investigation and conclude exactly how fast the car was going and whether any mechanical problems contributed to the crash. He noted that the coroner's office was awaiting dental records before officially confirming who was in the car – or at the wheel.

Multiple witnesses say Rodas was driving when the pair went for a spin through an industrial area during a charity event. Antonio Holmes, who knew both victims, tells PEOPLE that he was one of the first at the scene, having raced over with two fire extinguishers. He said the men never switched positions and Walker was still in the passenger seat, wearing his seat belt.

Holmes, who does body work for high-end vehicles such as Rodas's limited-edition, 610-hp Porsche Carerra GT, says he and other car enthusiasts at the accident site expect investigators will determine that mechanical failure was the main cause of the accident.

"For that car to disintegrate the way it did, speed, of course [was a factor], but something went wrong and it put them at an angle to the curb," Holmes tells PEOPLE. "There's not a shadow of a doubt in my mind, it wasn’t driver error."

But finding those answers will be more difficult, Maag says, because "it's an exotic car, so it's not built like a normal car. And this one's been burnt and mangled."

He hopes to glean important information from the car's black box, but first they have to find it.

"Porsche seems to think it has one. They put them in all different locations [in the car]. We have to dig for it," Maag says.

Once they have the box, "We may have to send [it] to Germany to be downloaded, or perhaps we can plug it into another Porsche GT and be able to read it from that."

He says the black box "will give us the speed, whether they were wearing their seat belts, all kinds of information 25 seconds before the collision and 5 seconds after."

Also complicating the investigation were rumors that the Porsche was racing another car. Investigators explored that tip but found no evidence to back it up, L.A. County Sheriff's Sgt. Richard Pena tells PEOPLE. "That lead has been exhausted."